Duk (dug) in Tibetan language means 'poison' and has the connotation of a substance being harmful. In the Tibetan medical tradition, anything which adversely affects our system and causes suffering and pain can be referred to as a poison; for example, the afflictive emotions are labelled 'mental poisons'. The term duk is coined based on its harmful effects, just like medicine is called men (sman) because of it being beneficial. Tibetan medical practitioners know that mercury is highly toxic. Nevertheless, Tibetan medicine still uses it in the particular form of mercury sulphide as an important ingredient in some multi-compound medicines to treat severe diseases. This is possible because Tibetan pharmacology describes detailed methods of processing mercury, which are complex and long and demand the involvement of highly skilled people and a particular environment. The ability to transform a virtual toxin into a tonic is an invaluable skill based on the in-depth understanding of the characteristics of the various types of duk contained in unprocessed mercury as this article will explore.